Add opt-out for rate limit model nudge

[aibrahim-oai]

Summary

• add a hide_rate_limit_model_nudge notice flag plus config edit plumbing so the rate limit reminder preference is persisted and documented
• extend the chat widget prompt with a "never show again" option, and wire new app events so selecting it hides future nudges immediately and writes the config
• add unit coverage and refresh the snapshot for the three-option prompt

Testing

• just fmt
• just fix -p codex-tui
• just fix -p codex-core
• cargo test -p codex-tui
• cargo test -p codex-core (fails at exec::tests::kill_child_process_group_kills_grandchildren_on_timeout: grandchild process still alive)

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Codex Task

[aibrahim-oai]

@codex review this

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Bravo.

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